1. Field of the Invention
The present invention relates, to a knocking detecting system for detecting the presence of knocking an automobile engines on the basis of the signal of a combustion vibration from a knocking sensor. More particularly, it relates to a system and a method in which a knocking signal is A/D-converted by a microcomputer for the detection of knocking and the calculation of a background level.
2. Description of the Prior Art
For the control of the ignition timing or supercharging pressure in an automobile engine, a judgement signal of a knocking detecting device for optimum control. The knocking detecting device in this case operates, for example, such that a signal from a knock sensor is amplified with an appropriate gain, a knocking component is extracted by a filter, and the waveform of the knocking component is rectified with respect to its central value. Subsequently, one of the rectified signals is used for evaluating a background level conforming to a noise level, and the other signal is compared with the background level, thereby to judge the presence of the knocking. Since means for creating the device are chiefly made up of analog circuits, the device has a large number of problems in circuit area, cost, the changes of detection characteristics attributed to the aging of analog elements, the number of stages of a matching operation, etc. Meanwhile, microcomputers with high-speed A/D converters have come into wide use in recent years, and it has been requested that the knocking detection based on the analog circuit processing should be performed by microcomputer processing.
Heretofore, as to the knocking detecting device of this type, there has been a prior-art technique disclosed in Japanese Patent application Laid-open No. 193333/1984 by way of example. Here, the following is indicated: A pressure signal from an intracylindrical pressure sensor is input to an integrator through a charge amplifier, a BPF (band-pass filter) and a rectifier. Subsequently, the pressure signal having been input is integrated over a predetermined angular extent before a top dead center in the signal of a crank angle and over the predetermined angular extent after the to dead center, and the ratio of the respective integral values is computed. The computed value is compared with a reference value conforming to operating conditions, thereby to judge knocking.
In addition, as to the setting of a background level, there is known a prior-art technique from Japanese Patent application Laid-open No. 79320/1987 by way of example. In this prior-art technique, the background level is calculated in terms of the average value of the levels of the knocking signals of an intracylindrical pressure sensor, and, when the signal level is predetermined times the background value or above, the renewal of the background value is suspended.
With the former of the prior-art techniques stated above, the pressure signal is integrated analogically, so that circuit arrangement and control become complicated. Besides, in such a case where the peak of combustion pressure is in the vicinity of the top dead center, it is sometimes impossible to detect the knocking precisely. With the latter of the prior-art techniques, the knocking signal of particularly high level is not referred to in the calculation of the background level. This such a problem that the level setting lacks preciseness.